Semiconductor device having thin film transistor with position controlled channel formation region

ABSTRACT

A semiconductor device production system using a laser crystallization method is provided which can avoid forming grain boundaries in a channel formation region of a TFT, thereby preventing grain boundaries from lowering the mobility of the TFT greatly, from lowering ON current, and from increasing OFF current. Rectangular or stripe pattern depression and projection portions are formed on an insulating film. A semiconductor film is formed on the insulating film. The semiconductor film is irradiated with continuous wave laser light by running the laser light along the stripe pattern depression and projection portions of the insulating film or along the major or minor axis direction of the rectangle. Although continuous wave laser light is most preferred among laser light, it is also possible to use pulse oscillation laser light in irradiating the semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device constructed by asemiconductor film that has a crystal structure, and more specifically,to a semiconductor device using a thin film transistor whose activelayer is formed of a crystalline semiconductor film obtained throughcrystal growth on an insulating surface. The present invention alsorelates to a semiconductor device product ion system using laser light.

2. Description of the Related Art

In recent years, techniques for forming TFTs on a substrate have madegreat advancements and applications of TFTs to active matrix typesemiconductor display devices are being developed. In particular, TFTsformed of polycrystalline semiconductor films (hereinafter referred toas polysilicon TFT) have higher field effect mobility (also referred toas mobility) than conventional TFTs that use amorphous semiconductorfilms, and accordingly can operate at high speed. Therefore pixels canbe controlled by a driving circuit formed on the same substrate on whichthe pixels are formed, instead of a driving circuit external to thesubstrate as with the conventional technique.

Incidentally, for substrates used in semiconductor devices, a glasssubstrate is deemed more promising than a single crystal siliconsubstrate cost-wise. Glass substrates have poor heat-resistance and areeasily deformed by heat. Therefore, when forming a polysilicon TFT on aglass substrate, using laser annealing to crystallize a semiconductorfilm in order to avoid thermal deformation of the glass substrate isextremely effective.

Laser annealing has characteristics such as remarkable reduction ofprocessing time compared to an annealing method utilizing radiantheating or thermal conductive heating, and a semiconductor or asemiconductor film is selectively and locally heated so that a substrateis scarcely thermally damaged.

Note that the term “laser annealing” herein indicates a technique forrecrystallizing a damaged layer formed on a semiconductor substrate orin a semiconductor film and a technique for crystallizing asemiconductor film formed on a substrate. The term “laser annealing”also includes a technique that is applied to leveling or improvement ofa surface quality of the semiconductor substrate or the semiconductorfilm. Applicable laser oscillation devices are gas laser oscillationdevices represented by an excimer laser, and solid laser oscillationdevices represented by a YAG laser. Such laser oscillation devices areknown to heat a surface layer of a semiconductor by laser beamirradiation for an extremely short period of time, i.e., about severaltens of nanoseconds to several tens of microseconds so as to crystallizethe surface layer.

Lasers are roughly divided into two types, pulse oscillation andcontinuous wave, by their oscillation methods. Pulse oscillation lasersare relatively high in output energy and therefore the size of laserbeam can be set to several cm² to increase the mass-productivity. Inparticular, if the shape of laser beam is processed by an optical systeminto a linear shape 10 cm or more in length, a substrate can beirradiated with the laser light efficiently to increase themass-productivity even more. Accordingly, using pulse oscillation lasersto crystallize semiconductor films have been becoming mainstream.

In recent years, however, it has been found that the grain size ofcrystals formed in a semiconductor film is larger when a continuous wavelaser is used to crystallize a semiconductor film than when a pulseoscillation laser is used. With crystals of larger grain size in asemiconductor film, the mobility of TFTs formed from this semiconductorfilm is increased while fluctuation in characteristics between the TFTsdue to grain boundaries is reduced. As a result, continuous wave lasersare now suddenly attracting attention.

Trying to form a single crystal semiconductor film on an insulatingsurface is not new and a technique called graphoepitaxy has been devisedas a more positive attempt. Graphoepitaxy is a technique in which alevel difference is formed on a surface of a quartz substrate, anamorphous semiconductor film or a polycrystalline semiconductor film isformed on the substrate, and the film is heated by a laser beam or aheater so that an epitaxial growth layer is formed with the leveldifference on the quartz substrate as the nucleus. This technique isdisclosed in, for example, Non-patent Literature 1.

[Non-Patent Literature 1]

J. Vac. Sci. Technol., “Grapho-epitaxy of silicon on fused silica usingsurface micropatterns and laser crystallization”, 16(6), 1979, pp.1640–1643.

Another semiconductor film crystallizing technique called graphoepitaxyis disclosed in, for example, Non-patent Literature 2. The literature isabout inducing epitaxial growth of a semiconductor film byartificially-created surface relief grating on an amorphous substrate.According to the graphoepitaxy technique disclosed in Non-patentLiterature 2, a level difference is formed on a surface of an insulatingfilm, a semiconductor film is formed on the insulating film, and thesemiconductor film is subjected to heating, laser light irradiation, orthe like to start epitaxial growth of crystals of the semiconductorfilm.

[Non-Patent Literature 2]

M. W. Geis, et al., “CRYSTALLINE SILICON ON INSULATORS BYGRAPHOEPITAXY”, Technical Digest of International Electron DevicesMeeting, 1979, p. 210.

Crystalline semiconductor films formed using laser annealing methods,which are roughly classified into pulse oscillation and continuous wave,are masses of crystal grains in general. These crystal grains havevarying sizes and are positioned at random, and it is difficult tospecify the position and size of crystal grains in forming a crystallinesemiconductor film. Therefore an active layer formed by patterning thecrystalline semiconductor film into islands generally have interfacebetween crystal grains (grain boundaries).

Unlike the inside of a crystal grain, a grain boundary has an infinitenumber of re-combination centers and trap centers due to an amorphousstructure and crystal defects. When carriers are trapped in these trapcenters, the potential of the grain boundary rises to block carriers andlower the current carrying characteristic of carriers. Therefore, grainboundaries in an active layer, in particular, in a channel formationregion of a TFT, seriously affect TFT characteristics by lowering themobility of the TFT greatly, by lowering ON current, and by increasingOFF current since a current flows in grain boundaries. Grain boundariesalso cause fluctuation in characteristic among TFTs that are intended tohave the same characteristic because the characteristic of a TFT havinggrain boundaries in its active layer is different from that of a TFTwhose active layer has no grain boundaries.

Crystal grains obtained by irradiating a semiconductor film with laserlight have varying sizes and are positioned randomly because of thefollowing reason. It takes time for a liquefied semiconductor film thathas been thoroughly melted by laser light irradiation to create a solidnucleus. As time passes, an infinite number of crystal nuclei aregenerated in the thoroughly melted region and crystals grow from thecrystal nuclei. Since positions of the crystal nuclei to be generatedare at random, they are distributed unevenly. Crystal growth is stoppedas crystal grains collide against each other. Accordingly, the crystalgrains obtained have varying sizes and are positioned at random.

Ideally, a channel formation region, which has a great influence overTFT characteristics, is formed from a single crystal grain removingadverse effect of grain boundaries. However, prior art is mostlyunsuccessful in forming a crystalline silicon film with no grainboundaries by laser annealing. Therefore no TFT whose active layer isformed of a crystalline silicon film crystallized by laser annealing hassucceeded in obtaining characteristics that rival the characteristics ofa MOS transistor manufactured on a single crystal silicon substrate.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedproblems, and an object of the present invention is therefore to providea semiconductor device production system using a laser crystallizationmethod which can avoid forming grain boundaries in a channel formationregion of a TFT, thereby preventing grain boundaries from lowering themobility of the TFT greatly, from lowering ON current, and fromincreasing OFF current.

The inventors of the present invention have found that, when asemiconductor film is formed on an insulating film having depression andprojection portions and is irradiated with laser light, crystal nucleiare generated in the vicinity of edges of the depression bottom orprojection top of the insulating film and crystal growth starts from thecrystal nuclei and proceeds in the direction parallel to the insulatingfilm. A depression portion refers to a dented region where no projectionportion is formed.

This mechanism is explained with reference to FIGS. 1A and 1B. FIG. 1Ashows a semiconductor film 11 formed on an insulating film 10 that has aprojection portion 10 a. The semiconductor film 11 is melted by laserlight irradiation and the heat in the semiconductor film 11 is releasedto the insulating film 10. The heat radiation is efficient where a largearea of the semiconductor film is in contact with the insulating film.For instance, in FIG. 1A, the heat is released to the insulating filmmore efficiently and crystal nuclei are formed faster in portions 14 and15 where the semiconductor film 11 and the insulating film 10 meet ontwo planes that intersect with each other than in portions 12 and 13where the semiconductor film 11 and the insulating film 10 meet on oneplane. Also, the heat radiation is efficient in a portion of theinsulating film that has a large heat capacitance. For example, theportion 14, which is in the vicinity of the edge of the depressionportion, is larger in volume of the insulating film within a certainrange and accordingly has larger heat capacitance than the portion 15,which is in the vicinity of the edge of the projection portion.Therefore released heat does not stay long in the portion 14 and heatradiation is more efficient in the portion 14 than in the portion 15. Asa result, crystal nuclei are formed faster in the portion 14 which isnear the edge of the depression portion than in the portion 15 which isnear the edge of the projection portion.

As time passes, crystal growth starts from the crystal nuclei formed inthe portion 14 near the edge of the depression portion and proceeds inthe direction parallel to the insulating film. Crystal growth directionsin a semiconductor film will be described with reference to FIG. 1B.FIG. 1B shows a semiconductor film 11 formed on an insulating film 10that has two projection portions 10 a and 10 b. In the semiconductorfilm 11, crystal growth starts from a portion 14 near an edge of adepression portion and proceeds in the upper and lateral directionsindicated by arrows. The crystal growth started from the portion 14toward lateral direction near an edge of a depression portion comes toan end as it meets crystal growth started from an edge of the adjacentdepression portion 14 halfway, thereby forming a grain boundary 16.

As described above, crystallization by laser light irradiation of aninsulating film that has a projection portion makes it possible tocontrol the position at which a grain boundary is formed to a certaindegree. This gives foresight of where grain boundaries are formed in thesemiconductor film at the stage of designing the shape of the insulatingfilm. In other words, the present invention can choose where grainboundaries are to be formed and this makes it possible to place achannel formation region, preferably an active layer, such that theactive layer or channel formation region includes as few grainboundaries as possible.

Specifically, the insulating film is given rectangular or stripe patterndepression and projection portions. Then a semiconductor film is formedon the insulating film and is irradiated with continuous wave laserlight along the stripe pattern depression and projection portions of theinsulating film or along the major or minor axis direction of therectangular. Although continuous wave laser light is most preferredamong laser light, it is also possible to use pulse oscillation laserlight in irradiating the semiconductor film. The projection portion insection in the direction perpendicular to the laser light scanningdirection may be rectangular, triangular, or trapezoidal.

A grain boundary is formed in the semiconductor film about the midpointbetween edges of adjacent projection portions and another grain boundaryis formed in the semiconductor film about the midpoint between edges ofa depression portion. These grain boundaries are formed by collisionbetween growing crystals. Accordingly, the present invention uses as achannel formation region a portion 17 between one edge of a depressionportion and the midpoint between the one edge and the other edge of thedepression portion, where fewer grain boundaries are formed to give theportion excellent crystallinity. A portion 18, which is between one edgeof a projection portion and the midpoint between the one edge and theother edge of the projection portion, has fewer grain boundaries to givethe portion excellent crystallinity and can also be used as an activelayer or a channel formation region. It is not that the excellentcrystallinity portions 17 and 18 have no grain boundaries. However, theportions 17 and 18 have better crystallinity even if they have grainboundaries because their crystal grains are large in size.

In the present invention, a semiconductor film crystallized by laserlight is patterned to remove a portion of the film around the midpointbetween edges of a depression portion or projection portion. Theremaining portion between one edge of a depression portion or projectionportion and the midpoint between the one edge and the other edge of thedepression portion or projection portion, which has fewer grainboundaries and therefore has excellent crystallinity, is used as anactive layer of a TFT. This makes it possible to avoid forming a grainboundary in a channel formation region of a TFT, thereby preventinggrain boundaries from lowering the mobility of the TFT greatly, fromlowering ON current, and from increasing OFF current. How far from anedge of a depression portion or projection portion is to be removed bypatterning can be decided at designer's discretion.

In general, laser beam edges and the vicinity thereof are lower inenergy density than the center of the laser beam and a semiconductorfilm irradiated with laser beam edges often has poor crystallinity. Itis therefore desirable at the time of laser light scanning to preventedges of laser light track from overlapping a portion that later servesas a channel formation region of a TFT.

To achieve this, a semiconductor device production system of the presentinvention first stores data of the shape of the insulating film orsemiconductor film viewed from above the substrate (pattern information)as the data is obtained in the design stage. From the patterninformation and the width of a laser beam in the direction perpendicularto the laser light scanning direction, the laser light scanning path isdetermined so that edges of the laser light track is prevented fromoverlapping at least a portion that serves as a channel formation regionof a TFT. Then the substrate is positioned with a marker as thereference and the semiconductor film on the substrate is irradiated withlaser light by running it along the scanning path determined.

The above-mentioned structure makes it possible to at least run laserlight over only portions that need laser light irradiation, instead ofirradiating the entire substrate with laser light. Therefore time forlaser irradiation of portions that do not need laser light irradiationcan be saved to shorten the whole laser irradiation time and improve thesubstrate processing speed. The above-mentioned structure also makes itpossible to avoid damage to a substrate which is caused by irradiating aportion that does not need laser irradiation with laser light.

The marker may be formed by directly etching the substrate with laserlight or the like, or may be formed in a part of the insulating filmhaving depression and projection portions at the same time theinsulating film is formed. Another method of positioning the substrateis to use an image pickup device such as a CCD to read the shape of theinsulating film or semiconductor film actually formed, then store it asdata in the first storing means, store in the second storing means theinsulating film or semiconductor film pattern information obtained inthe design stage, and check the data stored in the first storing meansagainst the pattern information stored in the second storing means.

By forming a marker in a part of the insulating film or by using theshape of the insulating film as a marker, one fewer marker mask isneeded and the marker can be formed and positioned more accurately thanwhen forming it on a substrate by laser light. As a result, thepositioning accuracy is improved.

In general, the energy density of laser light is not thoroughly uniformand is varied between different points in a laser beam. The presentinvention requires to irradiate at least an area that serves as achannel formation region, preferably the entire flat face of adepression portion or the entire flat face of a projection portion, withlaser light having a constant energy density. Therefore, it is necessaryin the present invention to use a laser beam having such an energydensity distribution that makes a region of the laser beam that hasuniform energy density completely overlap at least an area that servesas a channel formation region, preferably the entire flat face of adepression portion or the entire flat face of a projection portion,during laser light scanning. A shape desirable for a laser beam to meetthe above-mentioned energy density condition would be rectangular,linear, etc.

A slit may be used to cut off a portion of a laser beam that is low inenergy density. The use of a slit makes uniform crystallization possibleby irradiating the entire flat face of a depression portion or theentire flat face of a projection portion with laser light that hasrelatively uniform energy density. In addition, the use of a slit allowsa laser beam to partially change its width in accordance with theinsulating film or semiconductor film pattern information. This reduceslimitations in layout of a channel formation region or active layer of aTFT. The laser beam width here means the length of a laser beam in thedirection perpendicular to the scanning direction.

One laser beam obtained by synthesizing laser beams that are emittedfrom plural laser oscillators may be used in laser crystallization. Thisstructure allows low energy density portions of laser beams tosupplement one another.

After the semiconductor film is formed, the semiconductor film may becrystallized by laser light irradiation without exposing the film to theair (for example, noble gas, nitrogen, oxygen, or other specific gasatmosphere or a reduced pressure atmosphere is employed). This structurecan prevent molecule-level contaminants in a clean room, such as boroncontained in a filter for enhancing the cleanliness of the air, frommixing in the semiconductor film during laser light crystallization.

A conventional semiconductor film crystallization technique calledgraphoepitaxy is to induce epitaxial growth of a semiconductor film byartificially-created surface relief grating on an amorphous substrate.Graphoepitaxy-relating techniques are described in Non-patent Literature2 given in the above and others. The paper discloses that agraphoepitaxy technique is for forming a level difference on a surfaceof an insulating film, forming a semiconductor film on the insulatingfilm, and subjecting the semiconductor film to treatment such as heatingor laser light irradiation for epitaxial growth of crystals in thesemiconductor film. As the temperature required for epitaxial growth is700° C. or higher a glass substrate can not be used due to poor heatresisting properties. Even when epitaxial growth is attempted using aquartz substrate, a grain boundary is formed in the semiconductor filmnear center of a depression portion or projection portion of theinsulating film. In the present invention, the crystallinity of an areato form an island is improved by placing a mask for the island, so thatlayout of the island dictates the shape of a depression portion orprojection portion of the insulating film and the position of an edge ofa depression portion or projection portion. Specifically, the shape,size, and the like of a depression portion or projection portion aredetermined such that an island does not overlap an edge of thedepression portion or projection portion or the midpoint between theedges of the depression portion or projection portion. Using theinsulating film designed in accordance with the layout of the island,the position of a grain boundary is selectively set. A portion of thesemiconductor film where a grain boundary is selectively formed isremoved by patterning and the remaining portion, which has relativelygood crystallinity, is used as the channel formation region. Thetechnique disclosed in the present invention is similar to conventionalgraphoepitaxy in that a semiconductor film is formed on an insulatingfilm having a level difference and the level difference is used tocrystallize the semiconductor film. However, conventional graphoepitaxydoes not include using the level difference to control the position of agrain boundary and reduce grain boundaries in number in an island, andtherefore is not identical with the present invention despite theresemblance.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams showing crystal growth directions in asemiconductor film when irradiated with laser light;

FIGS. 2A to 2C are diagrams showing a semiconductor film beingirradiated with laser light;

FIGS. 3A to 3C are diagrams of islands formed by patterning acrystallized semiconductor film;

FIGS. 4A and 4B are diagrams showing the structure of a TFT that isformed from the islands shown in FIGS. 3A to 3C;

FIG. 5 is a flowchart of a production system of the present invention;

FIG. 6 is a diagram of laser irradiation apparatus;

FIG. 7 is a diagram of laser irradiation apparatus;

FIGS. 8A to 8D are diagrams showing a method of forming an insulatingfilm that has depression and projection portions;

FIGS. 9A to 9C are diagrams showing a method of forming an insulatingfilm that has depression and projection portions;

FIGS. 10A to 10C are diagrams of TFTs formed from islands that areseparated from each other;

FIGS. 11A and 11B are diagrams showing shapes of an insulating film thathas depression and projection portions;

FIGS. 12A to 12D are a top view and sectional views of a TFT that isformed from the insulating film shown in FIG. 11B;

FIGS. 13A to 13D are diagrams showing a method of manufacturing asemiconductor device using the present invention;

FIG. 14 is a diagram showing a method of manufacturing a semiconductordevice using the present invention;

FIGS. 15A to 15E are diagrams showing a method of crystallizing asemiconductor film using a catalytic metal;

FIGS. 16A and 16B are diagrams showing the energy density distributionof a laser beam;

FIGS. 17A and 17B are diagrams showing the energy density distributionof a laser beam;

FIG. 18 is a diagram showing the energy density distribution of a laserbeam;

FIG. 19 is a diagram of an optical system;

FIGS. 20A to 20C are diagrams of optical systems;

FIG. 21 is a diagram showing the energy density distribution in thecentral axis direction of laser beams overlapped;

FIG. 22 is a diagram showing the energy difference in relation to thedistance between the centers of laser beams;

FIG. 23 is a diagram showing the output energy distribution in thecentral axis direction of a laser beam;

FIG. 24 is a diagram showing the structure of a light emitting devicethat is an example of a semiconductor device of the present invention;

FIG. 25 is a diagram showing a pixel structure in a light emittingdevice that is an example of a semiconductor device of the presentinvention;

FIGS. 26A to 26H are diagrams of electronic equipment using asemiconductor device of the present invention;

FIG. 27 is a sectional view of TFTs forming a stack structure;

FIG. 28 is a diagram showing the energy density distribution of a laserbeam that is obtained by synthesizing two laser beams;

FIG. 29 is a diagram showing the energy density distribution of a laserbeam that is obtained by synthesizing four laser beams;

FIG. 30 is a diagram showing the energy density distribution of a laserbeam that is obtained by synthesizing four laser beams;

FIG. 31 shows the concentration profile of oxygen in a silicon filmcrystallized by laser light;

FIG. 32 shows the concentration profile of nitrogen in a silicon filmcrystallized by laser light;

FIG. 33 shows the concentration profile of carbon in a silicon filmcrystallized by laser light;

FIG. 34 shows the concentration profile of boron in a silicon filmcrystallized by laser light; and

FIGS. 35A to 35C are sectional views of an insulating film that hasdepression and projection portions and a semiconductor film that isformed on the insulating film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A laser light irradiation method used in the present invention will bedescribed with reference to FIGS. 2A to 2C.

First, an insulating film 101 is formed on a substrate 100 as shown inFIG. 2A. The insulating film 101 has projection portions 101 a forming astripe pattern. How the insulating film is given depression andprojection will be described later in detail. The insulating film 101 isa silicon oxide film, a silicon oxynitride film, a silicon nitride film,or the like. Other insulating films can be used as long as they areinsulating films which can prevent an alkaline metal or other impuritiesfrom entering a semiconductor film subsequently formed, which haveenough heat resistance to withstand the temperature in subsequenttreatment, and which can have depression and projection. The insulatingfilm 101 may also be a laminate of two or more films.

A marker may be formed from a part of the insulating film 101 at thesame time the insulating film 101 is formed.

The material of the substrate 100 has to have enough heat resistance towithstand the temperature in subsequent treatment. For example, a quartzsubstrate, silicon substrate, glass substrate, metal substrate, orstainless steel substrate with an insulating film formed on its surfaceis used as the substrate 100. The glass substrate is formed of bariumborosilicate glass, alumino-borosilicate glass, or the like. A plasticsubstrate may also be used if it has enough heat resistance to withstandthe temperature in subsequent treatment.

Next, a semiconductor film 102 is formed to cover the insulating film101. The semiconductor film 102 can be formed by a known method(sputtering, LPCVD, plasma CVD, or the like). The semiconductor film maybe an amorphous semiconductor film, a microcrystalline semiconductorfilm, or a crystalline semiconductor film. The semiconductor film mayalso be formed of silicon or silicon germanium.

The semiconductor film 102 also has depression and projection along thedepression and projection of the insulating film 101. The size of theprojection portions 101 a of the insulating film 101 can be set atdesigner's discretion but the projection portions have to be thickenough to avoid discontinuity in the subsequently-formed semiconductorfilm near edges of the projection portions. If an active layer is placedin a depression portion, restrictions in layout of an active layer canbe reduced by setting the depression portion wider than the projectionportion. If an active layer is placed in a projection portion,restrictions in layout of an active layer can be reduced by setting theprojection portion wider than the depression portion. In thisembodiment, the flat portion of a depression portion is twice wider thanthe flat portion of a projection portion or more and the width of aprojection portion is set to 300 to 3000 nm. The height of a projectionportion is set to 30 to 300 nm.

Next, the semiconductor film 102 is irradiated with laser light as shownin FIG. 2A to form a semiconductor film (post-LC) 103 with improvedcrystallinity. The laser light energy density is low in the vicinity ofthe edges of a laser beam 104. Therefore a film irradiated with thelaser beam edges has small crystal grains and a ridge is formedprotruding along a grain boundary. Therefore, the edges of the track ofthe laser beam 104 is prevented from overlapping a portion to serve as achannel formation region.

The laser light scanning direction is set parallel to the direction ofthe projection portions 101 a as indicated by the arrow.

The present invention can employ known lasers. Continuous wave laserlight is desirable but it is considered that pulse oscillation laserlight can also provide the effect of the present invention to a certaindegree. A gas laser or solid-state laser can be employed. Examples ofthe gas laser include an excimer laser, an Ar laser, and a Kr laser.Examples of the solid-state laser include a YAG laser, a YVO₄ laser, aYLF laser, a YAlO₃ laser, a glass laser, a ruby laser, an alexandritelaser, a Ti: sapphire laser, and a Y₂O₃ laser. The solid-state laseremployed is a laser that uses crystals of YAG, YVO₄, YLF, YAlO₃ or thelike doped with Cr, Nd, Er, Ho, Ce, Co, Ti, Yb, or Tm. The fundamentalwave of the laser is varied depending on the material used for doping,but laser light obtained has a fundamental wave of about 1 μm. Anon-linear optical element is used to obtain harmonic of the fundamentalwave.

Ultraviolet laser light may also be employed. The ultraviolet laserlight is obtained by using a non-linear optical element to convertinfrared laser light that is emitted from a solid-state laser into greenlaser light and then using another non-linear optical element to convertthe green laser light.

FIG. 2B corresponds to a sectional view taken along the line A-A′ ofFIG. 2A before crystallization and FIG. 2C is a sectional view takenalong the line B–B′ after crystallization. In the semiconductor film(post-LC) 103 crystallized by laser light irradiation, a grain boundary105 is easily formed around the center of a depression portion of theinsulating film 101. Used in FIGS. 2A to 2C as an active layer or achannel formation region is a portion 106 between one edge of adepression portion and the midpoint between the one edge and the otheredge of the depression portion, where fewer grain boundaries are formedto give the portion excellent crystallinity. It is not that theexcellent crystallinity portion 106 has no grain boundaries. However,the portion 106 has better crystallinity even if it has grain boundariesbecause its crystal grains are large in size. A portion between one edgeof a projection portion and the midpoint between the one edge and theother edge of the projection portion has fewer grain boundaries to givethe portion excellent crystallinity, and this portion too can be used asan active layer or a channel formation region.

The semiconductor film 103 after crystallization is then patterned asshown in FIG. 3A, avoiding the semiconductor film around the midpointbetween edges of a depression portion and near a projection portionwhere many grain boundaries are supposedly formed to be used as channelformation regions. The remaining portion, between one edge of adepression portion and the midpoint between the one edge and the otheredge of the depression portion, has excellent crystallinity and is usedto form channel formation regions.

In this embodiment, the semiconductor film 103 is patterned so as topartially leave regions near edges of projection portions or regionsnear edges of depression portions, projection portions, and regionsaround the centers of depression portions. Thus obtained is the island108 in which only channel formation regions are separated as shown inFIG. 3A and which is used as a slit-like active layer. A sectional viewtaken along the line A–A′ of the island 108 is shown in FIG. 3B and FIG.3C is a sectional view taken along the line B–B′ of the island 108. In aportion to serve as a source region or a drain region, the crystallinityof a semiconductor film has less influence over TFT characteristics thanin a channel formation region. Therefore using as a source region or adrain region a portion of a semiconductor film that has rather poorcrystallinity does not present a problem.

Next, a gate insulating film 110 is formed as shown in FIG. 4A to coverat least a portion of the island 108 that serves as a channel formationregion. Although a portion to serve as a source region or a drain regionis exposed in FIG. 4A, the gate insulating film 110 may cover the entireisland 108.

Then a conductive film is formed and patterned to form a gate electrode111. A sectional view taken along the line A–A′ in FIG. 4A is shown inFIG. 4B. The gate electrode 111 overlaps every channel formation region.

Through the above-mentioned manufacturing process, a TFT having channelformation regions separated from one another is completed. Thisstructure makes it possible to increase the channel width in a channelformation region so that the TFT can be driven while ensuring ONcurrent. As a result, the heat generated can be released efficiently.

The description given next is about a production system of the presentinvention. FIG. 5 is a flow chart for a production system of the presentinvention. First, a mask for an island is designed and then aninsulating film is designed to have rectangular or stripe patterndepression and projection portions. One or more channel formationregions are placed on the flat face of a depression portion orprojection portion of the insulating film. Desirably, a channelformation region is placed avoiding a region around the midpoint betweenedges of a depression portion or projection portion where more grainboundaries are formed than the rest. The carrier moving direction in achannel formation region desirably matches the direction of the stripepattern of the insulating film, or the direction of the longer sides orshorter sides of the rectangular of the insulating film. However, theymay be varied intentionally if it suits the use.

The insulating film may be designed to have a marker as its part.

Information relating to the shape of the insulating film designed(pattern information) is inputted to a computer of laser irradiationapparatus and stored in storing means of the computer. The computerdecides the laser light scanning path based on the insulating filmpattern information inputted and the width in the directionperpendicular to the laser beam scanning direction. It is important indetermining the scanning path that the edges of the laser light track donot overlap the flat face of a depression portion or projection portionof the insulating film. The computer may store in its storing meanspattern information of an island in addition to the insulating filmpattern information, and may decide the scanning path so as to preventthe edges of the laser light track from overlapping the island or achannel formation region of the island.

If a slit is used to control the width of a laser beam, the computergrasps the width of a depression portion or projection portion of theinsulating film in the direction perpendicular to the scanning directionfrom the insulating film pattern information inputted. Then, taking intoaccount the width of a depression portion or projection portion of theinsulating film, the width of the slit in the direction perpendicular tothe scanning direction is set so as to prevent the edges of the laserlight track from overlapping the flat face of a depression portion orprojection portion of the insulating film.

After the insulating film is formed on a substrate in accordance withthe designed pattern information, a semiconductor film is formed on theinsulating film. After the semiconductor film is formed, the substrateis set on a stage of the laser irradiation apparatus and is positioned.In FIG. 5 illustrates an example in which the substrate is positioned bydetecting the marker with a CCD camera. A CCD camera refers to a camerausing a CCD (charge-coupled device) as an image pickup device.

In another method to position the substrate, a CCD camera or the like isused to detect pattern information of the insulating film orsemiconductor film on the substrate that is set on the stage, and thenthe computer checks the pattern information of the insulating film orsemiconductor film actually formed on the substrate which is provided bythe CCD camera against information of an insulating film orsemiconductor film pattern designed by CAD.

Laser light irradiates the semiconductor film by running along thescanning path determined and crystallizes the semiconductor film.

The semiconductor film having its crystallinity enhanced by the laserlight irradiation is patterned to form an island. Subsequently, aprocess of manufacturing a TFT from the island follows. Althoughspecifics of the TFT manufacturing process are varied depending on theTFT form, a typical process starts with forming a gate insulating filmand forming impurity regions in the island. Then an interlayerinsulating film is formed so as to cover the gate insulating film and agate electrode. A contact hole is formed in the interlayer insulatingfilm to partially expose the impurity region. A wire is then formed onthe interlayer insulating film to reach the impurity region through thecontact hole.

Next, a description is given with reference to FIG. 6 on the structureof laser irradiation apparatus used in the present invention. Referencesymbol 151 denotes laser oscillators. Four laser oscillators are used inFIG. 6 but the number of laser oscillators in the laser irradiationapparatus is not limited thereto.

A chiller 152 may be used to keep the temperature of the laseroscillators 151 constant. Although the chiller 152 is not alwaysnecessary, fluctuation in energy of laser light outputted due to atemperature change can be avoided by keeping the temperature of thelaser oscillators 151 constant.

Denoted by 154 is an optical system, which changes the path of lightemitted from the laser oscillators 151 or manipulates the shape of thelaser beam thereof to collect laser light. In the laser irradiationapparatus of FIG. 6, the optical system 154 can also synthesize laserbeams of laser light outputted from the plural laser oscillators 151 bypartially overlapping the laser beams.

An AO modulator 153 capable of changing the travel direction of laserlight in a very short time may be provided in the light path between asubstrate 156 that is a processing object and the laser oscillators 151.Instead of the AO modulator, an attenuator (light amount adjustingfilter) may be provided to adjust the energy density of laser light.

Alternatively, energy density measuring means 165, namely, means formeasuring the energy density of laser light outputted from the laseroscillators 151 may be provided in the light path between the substrate156 that is a processing object and the laser oscillators 151. Changeswith time of measured energy density may be monitored by a computer 160.In this case, output from the laser oscillators 151 may be increased tocompensate attenuation in energy density of the laser light.

A synthesized laser beam irradiates through a slit 155 the substrate 156that is a processing object. The slit 155 is desirably formed of amaterial that can block laser light and is not deformed or damaged bylaser light. The width of the slit 155 is variable and a laser beam canbe changed in width by changing the width of the slit.

When laser light emitted from the laser oscillators 151 does not passthrough the slit 155, the shape of the laser beam on the substrate 156is varied depending on the laser type and may be shaped by an opticalsystem.

The substrate 156 is set on a stage 157. In FIG. 6, position controllingmeans 158 and 159 correspond to means for controlling the position of alaser beam on a processing object. The position of the stage 157 iscontrolled by the position controlling means 158 and 159.

In FIG. 6, the position controlling means 158 controls the position ofthe stage 157 in the direction X and the position controlling means 159controls the position of the stage 157 in the direction Y.

The laser irradiation apparatus of FIG. 6 has the computer 160, which isa central processing unit and at the same time storing means such as amemory. The computer 160 controls oscillation of the laser oscillators151, determines the laser light scanning path, and controls the positioncontrolling means 158 and 159 to move the substrate to a given positionso that a laser beam runs along the scanning path determined.

In FIG. 6, the laser beam position is controlled by moving thesubstrate. Alternatively, the laser beam position may be moved by anoptical system such as a Galvano mirror. The laser beam position mayalso be controlled by moving both the substrate and the laser beam.

In FIG. 6, the computer 160 controls the width of the slit 155 so thatthe laser beam spot width can be changed in accordance with mask patterninformation. The slit is not always necessary.

The laser irradiation apparatus may also have means for adjusting thetemperature of a processing object. A damper may also be provided toprevent reflected light from irradiating a portion that should avoidlaser irradiation since laser light is highly directional and has highenergy density. Desirably, the damper is absorptive of reflected light.Cooling water may be circulated inside the damper to avoid a temperaturerise of the partition wall due to absorption of reflected light. Thestage 157 may be provided with means for heating a substrate (substrateheating means).

If a laser is used to form a marker, a laser oscillator for a marker maybe provided. In this case, oscillation of the laser oscillator for amarker may be controlled by the computer 160. Another optical system isneeded when the laser oscillator for a marker is provided in order tocollect laser light outputted from the laser oscillator for a marker.The laser used to form a marker is typically a YAG laser or a CO₂ laser,but it is needless to say that other lasers may be employed instead.

One, or more if it is necessary, CCD camera(s) 163 may be provided forpositioning that uses a marker. A CCD camera refers to a camera using aCCD (charge-coupled device) as an image pickup device.

Instead of forming a marker, the CCD camera(s) 163 may be used torecognize the pattern of the insulating film or semiconductor film forpositioning of the substrate. In this case, insulating film orsemiconductor film pattern information by a mask which is inputted tothe computer 160 and the actual insulating film or semiconductor filmpattern information collected by the CCD camera(s) 163 are checkedagainst each other to grasp the substrate position information.

Part of laser light entering the substrate is reflected by the surfaceof the substrate and travels back the same light path it has taken uponentering. This is called return light and has adverse effects such aschanging the output and frequency of the laser and damaging the rod. Inorder to remove such return light and stabilize laser oscillation, anisolator may be provided.

Although FIG. 6 shows a laser irradiation apparatus structure which hasplural laser oscillators, only one laser oscillator may be provided.FIG. 7 shows a laser irradiation apparatus structure which has one laseroscillator. In FIG. 7, 201 denotes a laser oscillator and 202 denotes achiller. Denoted by 215 is an energy density measuring device, 203, anAO modulator, 204, an optical system, 205, a slit, and 213, a CCDcamera. A substrate 206 is set on a stage 207. The position of the stage207 is controlled by X-direction position controlling means 208 andY-direction position controlling means 209. Similar to the apparatusshown in FIG. 6, a computer 210 controls operations of the means of thislaser irradiation apparatus. The major difference between FIG. 7 andFIG. 6 is that there is one laser oscillator in FIG. 7. Unlike FIG. 6,the optical system 204 only has to have a function of collecting onelaser beam.

As described above, in the present invention, a semiconductor filmcrystallized by laser light is patterned to remove a portion of the filmaround the midpoint between edges of a depression portion or projectionportion. The remaining portion between one edge of a depression portionor projection portion and the midpoint between the one edge and theother edge of the depression portion or projection portion, where fewergrain boundaries are formed to give the film excellent crystallinity, iseffectively used as a channel formation region of a TFT. This makes itpossible to avoid forming a grain boundary in a channel formation regionof a TFT, thereby preventing grain boundaries from lowering the mobilityof the TFT greatly, from lowering ON current, and from increasing OFFcurrent. How far from an edge of a depression portion or projectionportion is to be removed by patterning can be decided at designer'sdiscretion.

The present invention runs laser light so as to obtain at least theminimum degree of crystallization of a portion that has to becrystallized, instead of irradiating the entire semiconductor film withlaser light. As a result, time for laser irradiation of portions thatare removed by patterning after crystallization of the semiconductorfilm can be saved to greatly shorten the processing time per substrate.

(Embodiment 1)

This embodiment explains how to form an insulating film havingdepression/projection.

At first, a first insulating film 251 is formed on a substrate 250, asshown in FIG. 8A. Although the first insulating film 251 uses siliconoxide nitride in this embodiment, this is not limited to, i.e. aninsulating film having a great etching selective ratio to a secondinsulating film is satisfactory. In this embodiment, the firstinsulating film 251 was formed to a thickness of 50–200 nm using SiH₄and N₂O by a CVD apparatus. Note that the first insulating film may beof a single layer or a layered structure having a plurality ofinsulating films.

Then, a second insulating film 252 is formed in contact with the firstinsulating film 251, as shown in FIG. 8B. The second insulating film 252requires a film thickness to a degree that, when a depression-projectionis formed thereon by patterning in a subsequent process, thedepression-projection appears on a surface of a semiconductor film to besubsequently deposited. This embodiment forms, as the second insulatingfilm 252, silicon oxide having 30 nm–300 nm by a plasma CVD.

Next, a mask 253 is formed as shown in FIG. 8C to etch the secondinsulating film 252. This embodiment conducts wet etching at 20° C.using an etchant of a mixture solution containing 7.13% of ammoniumhydrogen fluoride (NH₄HF₂) and 15.4% of ammonium fluoride (NH₄F)(product name: LAL500 by Stella Chemifa Corporation). This etching formsa projection part 254 in a rectangular or stripe form. In thisspecification, a combination of the first insulating film 251 and theprojection part 254 is considered as one insulating film. And then, themask 253 is removed.

Then, a semiconductor film is formed covering the first insulating film251 and projection part 253. Because in the embodiment the projectionpart has a thickness of 30 nm–300 nm, the semiconductor film isdesirably given a film thickness of 50–200 nm, herein 60 nm.Incidentally, in case an impurity is mixed between the semiconductorfilm and the insulating film, there is a possibility that bad affectionis exerted to the crystallinity of semiconductor film to increase thecharacteristic and threshold voltage variation of the TFT fabricated.Accordingly, the insulating film and the semiconductor film aredesirably formed continuously. For this reason, in this embodiment,after forming an insulating film comprising the first insulating film251 and the projection part 253, a silicon oxide film is formed in asmall thickness on the insulating film, followed by continuously forminga semiconductor film 256 without exposure to the air. The thickness ofsilicon oxide film, although properly set by the designer, was given 5nm–30 nm in this embodiment.

Incidentally, when etching the second insulating film 252, theprojection part may be etched into a taper form. By making theprojection part in a taper form, a semiconductor film, gate insulatingfilm or gate electrode is prevented from having disconnection at aprojection-region edge.

Now, explanation is made on a different way to form an insulating film.At first, a first insulating film 261 is formed on a substrate 260, asshown in FIG. 9A. The first insulating film 261 is formed of siliconoxide, silicon nitride or silicon oxide nitride.

In the case of using a silicon oxide nitride film, it can be formed bymixing Tetraethyl Ortho Silicate (TEOS) and O₂ and subjecting it to aplasma CVD with discharge under a reaction pressure of 40 Pa, at asubstrate temperature of 300–400° C. and with a radio frequency (13.56MHz) power density of 0.5–0.8 W/cm₂. In the case of using a siliconoxide nitride film, it may be formed by a plasma CVD with a siliconoxide nitride film formed from SiH₄, N₂O and NH₃ or a silicon oxidenitride film formed from SiH₄ and N₂O. This is performed under a formingcondition of a reaction pressure of 20–200 Pa and a substratetemperature of 300–400° C., with a radio frequency (60 MHz) powerdensity of 0.1–1.0 W/cm². Meanwhile, a silicon oxide nitride hydridefilm may be used that is to be formed from SiH₄, N₂O and H₂. A siliconnitride film can be similarly formed from SiH₄ and NH₃ by a plasma CVD.

After forming a first insulating film 261 to a thickness of 20 –200 nm(preferably 30–60 nm) over the entire surface of the substrate, a mask262 is formed by using a photolithography technique as shown in FIG. 9B.Unwanted regions are removed away to form a projection part 263 in astripe or rectangular form. To remove away unwanted regions, a dryetching process may be used that uses a fluorine-based gas. Otherwise, awet etching process may be used that uses a fluorine-based solution. Inthe case of selecting the latter, etching is preferably conducted usinga mixture solution containing 7.13% of ammonium hydrogen fluoride(NH₄HF₂) and 15.4% of ammonium fluoride (NH₄F) (product name: LAL500 byStella Chemifa Corporation). And then, the mask 262 is removed.

Then, a second insulating film 264 is formed covering the projectionpart 263 and substrate 260. This layer is formed of silicon oxide,silicon nitride or silicon oxide nitride to a thickness of 50–300 nm(preferably 100–200 nm), similarly to the first insulating film 261.

By the above fabrication process, an insulating film is formedcomprising the projection part 263 and the second insulating film 264.After forming the second insulating film 264, by continuously forming asemiconductor film without exposure to the air, the impurities in theair are prevented from mixing between the semiconductor film and theinsulating film.

(Embodiment 2)

This embodiment explains an example that a semiconductor film formed onan insulating film in a stripe form is crystallized by laser lightirradiation and thereafter mutually isolated islands are formed on asurface parallel with an projection-formed substrate to fabricate TFTusing the islands.

FIG. 10A shows a TFT structure of this embodiment. In FIG. 10A, aninsulating film 152 having striped projection parts 151 is formed on asubstrate 150. A plurality of islands 153 are formed, isolated from oneanother, on the top surfaces of the projection parts 151. An gateinsulating film 154 is formed in a manner contacting with the islands153. Incidentally, although the gate insulating film 154 in FIG. 10A isformed exposing the regions, to be made into impurity regions, of theisland, it may be formed covering the entire island 154.

A plurality of gate electrodes 155 is formed on the gate insulating film154 in a manner superposed over a plurality of islands 153. Theplurality of gate electrodes 155 may be mutually connected dependingupon a circuit configuration.

Note that the sectional view on the line A–A′ in FIG. 10A corresponds toFIG. 10B while the sectional view on the line B–B′ in FIG. 10Acorresponds to FIG. 10C. As shown in FIG. 10C, each gate electrode 155is superposed on a channel region 156 of the island 153 with gateinsulating film 154 sandwiched therebetween. The channel region 156, inturn, is sandwiched between two impurity regions 157 included also inthe island 153.

In this embodiment TFT is formed by using an island formed at the bottomof the depression part. TFT can also be formed by using an island on topof the projection part.

This embodiment can be implemented by combining with Embodiment 1.

(Embodiment 3)

This embodiment explains variations of insulating film forms.

FIG. 11A shows an embodiment on an insulating film form of theinvention. In FIG. 11A, an insulating film 171 is formed on a substrate170 wherein the insulating film 171 has a plurality of projection parts172. The projection part 172 is rectangular in form as viewed from theabove. All the projection parts have respective rectangular longer orshorter sides in a direction parallel with a scanning direction of laserlight shown by the arrow.

The projection parts 172 are not necessarily identical to one another inthe width in laser-light scanning direction and the width perpendicularto the scanning direction. A form of an insulating film is desirablydesigned to meet a desired island form.

It is not necessary that projection parts of the insulating film singthe present invention being completely striped. It is need only portionof the insulating film is striped or rectangular. FIG. 11B shows anembodiment on an insulating film form of the invention. In FIG. 11B, aninsulating film 181 is formed on a substrate 180. The insulating film181 is formed with a rectangular projection part 182 having slit-likeopenings as viewed from the above. In the projection part 182, the slithas a longer or shorter side in parallel with a scanning direction oflaser light shown by the arrow.

Explanation is now made on an example of a TFT structure formed by usingthe insulating film having slit-like openings shown in FIG. 11B.

FIG. 12A shows a top view of the TFT of this embodiment. As shown inFIG. 12A, this embodiment used an insulating film having a rectangularprojection part 760 having therein slit-like openings. A semiconductorfilm is formed covering the projection part 760. Laser light is scanned,in a direction shown by the arrow, along a direction of a longer axis ofthe slit-like opening to crystallize the semiconductor film. Then, thesemiconductor film is patterned to form an island 761 having an openingformed in the upper surface of the projection part. The channel regionof the island 761 avoids using the vicinity of a center between edges ofdepression part to use a portion having high crystallinity between theedge-neighborhood of a depression part and the vicinity of a centerbetween edges of a depression part.

Then, a gate insulating film 762 is formed in a manner contacting withthe island 761. Then, a conductive film is formed on the gate insulatingfilm 762. By patterning the conductive film, a gate electrode 763 isformed. The gate electrode 763 is superposed on a channel region 764 ofthe island 761 with gate insulating film 762 sandwiched therebetween.The channel region 764 is sandwiched between the two impurity regions765 included in the island 761.

A first interlayer insulating film 766 is formed covering the gateelectrode 763, island 761 and gate insulating film 762. The firstinterlayer insulating film 766 is formed of inorganic insulator havingan effect to prevent a substance, such as alkali metal, having a badeffect upon TFT characteristics from mixing in the island 761.

A second interlayer insulating film 767 is formed of organic resin onthe first interlayer insulating film 766. Openings are formed, byetching, through the second interlayer insulating film 767, firstinterlayer insulating film 766 and gate insulating film 762. Through theopenings, the interconnections 768, 769 are formed on the secondinterlayer insulating film 767, respectively connecting between the twoimpurity regions 765 and the gate electrode 763. Note that the sectionalview on the line A–A′ in FIG. 12A is shown in FIG. 12B, the sectionalview on the line B–B′ in FIG. 12C and the sectional view on the lineC–C′ in FIG. 12D.

In this embodiment, channel regions 764 are formed in plurality and thechannel regions are isolated from each other. Accordingly, by increasingthe channel width of the channel region, the heat generated by drivingthe TFT can be efficiently dissipated while securing on-current.

In this embodiment, TFT having a channel region formed at the bottom ofthe depression part is described though, it can also be formed by usingchannel region formed on top of the projection part.

(Embodiment 4)

This embodiment explains a method for manufacturing an active-matrixsubstrate using a laser crystallization method of the invention, byusing FIGS. 13 and 14. In this specification, the substrate forming, onthe same substrate, a CMOS circuit and a pixel region having drivecircuit, pixel TFTs and hold capacitances is referred to as anactive-matrix circuit, for ease of description.

This embodiment uses a substrate 600 formed of a glass such as bariumborosilicate glass or aluminum borosilicate glass. The substrate 600 mayuse a quartz, silicon, metal or stainless steel substrate formed with aninsulating film on a surface thereof. Otherwise, a plastic substrate maybe used that has a heat resistance to withstand at process temperaturein this embodiment.

Then, an insulating film of silicon oxide, silicon nitride or siliconoxide nitride is formed in a thickness of 100–300 nm on the substrate600, by the known means (a sputtering, an LPCVD, a plasma CVD or thelike).

Next, in order to form a large and small thickness regions in theinsulating film, the embodiment forms a resist mask 693 by aphotolithography technique and carries out an etching process on it.Although the dimension of a step is determined by an etching amount, theembodiment provides nearly 50–100 nm. For example, to etch a siliconoxide nitride film having 150 nm by 75 nm, it is possible to use wetetching using a solution containing hydrogen fluoride or applying a dryetching using CF₄. In this manner, an insulating film 601 formed with aprojection form is formed. In this case, the width of a projection partperpendicular to a scanning direction may be properly determined takinga TFT size into consideration, preferably a size (in diameter ordiagonal length) of approximately 2–6 μm for the purpose of controllingthe number of crystal-nucleation (FIG. 13A).

Then, an amorphous semiconductor film 692 is formed in a thickness of25–80 nm (preferably 30–60 nm) on the insulating film 601, by the knownmeans (sputter process, LPCVD process, plasma CVD process or the like)(FIG. 13B). Incidentally, although this embodiment forms an amorphoussemiconductor film, a fine crystal semiconductor film or crystallinesemiconductor film is also applicable. Otherwise, anamorphous-structured compound semiconductor film, such as an amorphoussilicon-germanium film, may be used.

Next, the amorphous semiconductor film 692 is crystallized by a lasercrystallization method. The scanning direction of laser light isparallel with an extension of the striped projection part of insulatingfilm 601. Incidentally, where the projection part of insulating film 601is rectangular as viewed from the above of the substrate, the scanningdirection of laser light is defined parallel with a direction of alonger or shorter side of the rectangle. Specifically, laser light ispreferentially irradiated according to the information about maskinputted to the computer of the laser irradiation apparatus. Of course,besides the laser crystallization method, this may be combined withother known crystallization methods (thermal crystallization methodusing RTA or furnace anneal, thermal crystallization method using ametal element to promote crystallization, or the like). Although theembodiment shows an example to change a laser beam width to a width ofinsulating film perpendicular to a scanning direction by the use of aslit, the invention is not limited to this, i.e. the slit is notnecessarily required to be used.

In crystallizing the amorphous semiconductor film, by using a continuousoscillatable solid laser and a second to fourth harmonic of basic wave,an increased grain size of crystal can be obtained. Typically, desirablyused is the second harmonic (532 nm) or third harmonic (355 nm) of anNd:YVO₄ laser (basic wave: 1064 nm). Specifically, the laser lightemitted from a continuous-oscillation YVO₄ laser is changed into aharmonic by a nonlinear optical device to obtain a 10 W-output laserlight. Meanwhile, there is a method that an YVO₄ crystal and a nonlinearoptical device are inserted in a resonator to emit a higher harmonic.Preferably, laser light is formed by an optical system into arectangular or elliptic form on irradiation plane, which is irradiatedto a subject to be worked. The energy density, in this case, requiresapproximately 0.01–100 MW/cm² (preferably 0.1–10 MW/cm²). Forirradiation the semiconductor film is moved at a speed of approximately10–2000 cm/s relatively to laser light.

In laser irradiation can be used a pulse-oscillation orcontinuous-oscillation gas laser or solid laser. Gas lasers includes anexcimer laser, an Ar laser and a Kr laser. Solid lasers include a YAGlaser, a YVO₄ laser, a YLF laser, a YAlO₃ laser, a glass laser, a rubylaser, an alexandorite laser, a Ti:sapphire laser and a Y₂O₃ laser. Asthe solid laser can be used a laser using a crystal of YAG, YVO₄, YLF orYAlO₃ doped with Cr, Nd, Er, Ho, Ce, Co, Ti Yb or Tm. Also, a slab laseris usable. The laser has a different basic wave depending upon a dopingmaterial, providing laser light having a basic wave at around 1 μm. Theharmonic to basic wave is available by the use of a non-linear opticaldevice.

The foregoing laser crystallization forms a crystalline semiconductorfilm 694 enhanced in crystallinity (FIG. 13C). In the crystallinesemiconductor film, grain boundaries 695 tend to occur in the vicinityof center between edges.

Next, the crystalline semiconductor film 694 enhance in crystallinity ispatterned into a desired form to form crystallized islands 602–606 (FIG.13D). Here, a number of grain boundaries in the islands 602 to 606 canbe suppressed by removing the vicinity of a center between an edge ofthe depression or projection part where the grain boundaries 695 tend tooccur.

After the islands 602 to 606 are formed, the islands may be doped with aminute amount of impurity element (boron or phosphorus) in order tocontrol the threshold of TFTs.

Subsequently, a process of manufacturing TFTs from the islands 602 to606 follows. Although specifics of the TFT manufacturing process arevaried depending on the TFT form, a typical process starts with forminga gate insulating film and forming impurity regions in the islands. Thenan interlayer insulating film is formed so as to cover the gateinsulating film and a gate electrode. A contact hole is formed in theinterlayer insulating film to partially expose the impurity regions. Awire is then formed on the interlayer insulating film to reach theimpurity regions through the contact hole.

FIG. 14 is a sectional view of the semiconductor device of thisembodiment. The islands 602 to 606 have channel formation regions, firstimpurity regions, and second impurity regions. Each channel formationregion is sandwiched between two first impurity regions. Each secondimpurity region is formed between one first impurity region and onechannel formation region. The concentration of an impurity element thatgives one conductivity type is higher in a first impurity region than ina second impurity region. A gate insulating film 607 is formed to coverthe islands 602 to 606. On the gate insulating film 607, gate electrodes608 to 613 are formed overlapping the channel formation regions and asource signal line 614 is also formed. An interlayer insulating film 615is formed on the gate insulating film 607 so as to cover the gateelectrodes 608 to 613 and the source signal line 614.

In a driving circuit 686, formed on the interlayer insulating film 615are wires 663 to 667., which are electrically connected to the impurityregions. In a pixel portion 687, a pixel electrode 670, a gate wire 669,and a wire 668 are formed. The wire 668 electrically connects the sourcesignal line 614 with a pixel TFT 684.

Although not shown in the drawing, the gate wire 669 is electricallyconnected to the gate electrodes 611 and 612 of the pixel TFT 684. Thepixel electrode 670 is electrically connected to a first impurity regionof the pixel TFT and is electrically connected to the island 606, whichfunctions as one of electrodes constituting capacitor storage 685. Inthis application, the pixel electrode and the wires are formed from thesame material. However, the pixel electrode 670 may be formed from ahighly reflective material such as a film mainly containing Al, a filmmainly containing Ag, or a laminate of films containing Al and Ag.

The driving circuit 686, which has a CMOS circuit composed of ann-channel TFT 681 and a p-channel TFT 682 and has an n-channel TFT 683,and the pixel portion 687, which has the pixel TFT 684 and the capacitorstorage 685, can thus be formed on the same substrate to complete anactive matrix substrate. The capacitor storage 685 is composed of theelectrode 613 and the island 606 with the gate insulating film 607 asdielectric.

According to the pixel structure of this embodiment, edges of a pixelelectrode overlap a source signal line so that the gap between pixelelectrodes is shielded against light without using a blocking film.

Although the structure of an active matrix substrate used in a liquidcrystal display device is described in this embodiment, themanufacturing process of this embodiment can also be employed tomanufacture a light emitting device. “Light emitting device” is ageneric term for a display panel in which a light emitting element isformed on a substrate and sealed between the substrate and a covermember, and for a display module obtained by mounting a TFT and the liketo the display panel. A light emitting element has a layer (lightemitting layer) containing an organic compound that providesluminescence upon application of electric field (electroluminescence),as well as an anode layer and a cathode layer.

In a light emitting element, a hole injection layer, an electroninjection layer, a hole transporting layer, an electron transportinglayer, or the like may be formed from an inorganic compound alone orfrom a material obtained by mixing an inorganic compound with an organiccompound. These layers may be partially blended with one another.

The present invention is also applicable to a semiconductor element ofsub-micron level. In the example shown in this embodiment, an activelayer is formed on the bottom of a depression portion. Alternatively, anactive layer may be formed on the top of a projection portion.

(Embodiment 5)

This embodiment explains an example of a combination of a laserirradiation process and a semiconductor film crystallizing process usinga catalyst when crystallizing a semiconductor film. In the case of usinga catalytic element, desirably used is an art disclosed inJP-A-130652/1995 and JP-A-78329/1996.

At first, as shown in FIG. 15A, an insulating film 501 having aprojection part 502 is formed on a substrate 500. Then, a semiconductorfilm 503 is formed on the insulating film 501.

Next, a catalytic element is used to crystallize the semiconductor film503 (FIG. 15B). For example, in the case of using the art disclosed inJP-A-130652/1995, a nickel acetate solution containing 10 ppm nickel byweight is applied onto the semiconductor film 503 to form anickel-containing layer 504. After a dehydrogenation process at 500° C.for 1 hour, thermal process is carried out at 500–650° C. for 4–12hours, e.g. at 550° C. for 8 hours, to form a semiconductor film 505enhanced in crystallinity. Incidentally, usable catalytic elements maybe an element of germanium (Ge), iron (Fe), palladium (Pd), tin (Sn),lead (Pb), cobalt (Co), platinum (Pt), copper (Cu) or Gold (Au), besidesnickel (Ni).

By laser irradiation, a semiconductor film 506 further enhanced incrystallinity is formed from the semiconductor film 505 crystallized bythe heat treatment using Ni. The semiconductor film 506 obtained bylaser irradiation contains a catalytic element. After laser irradiation,carried out is a process to remove the catalytic element from thesemiconductor film 506 (gettering). For gettering, it is possible to usean art described in JP-A-135468/1998 or JP-A-135469/1998.

Specifically, a phosphorus-added region 507 is formed in a part of asemiconductor film 506 obtained after laser irradiation. Thermal processis carried out in a nitrogen atmosphere at 550–800° C. for 5–24 hours,e.g. at 600° C. for 12 hours. Then, the phosphorus-added region 507 ofthe semiconductor film 506 acts as a gettering site to aggregate thecatalytic element existing in the semiconductor film 506 to thephosphorus-added region 507 (FIG. 15D).

Thereafter, the phosphorus-added region 507 of the semiconductor film506 is removed by patterning, thereby obtaining an island 508 reduced incatalytic element concentration to 1×10¹⁷ atoms/cm³ or less, preferablyto approximately 1×10¹⁶ atoms/cm³ (FIG. 15E).

Incidentally, after applying a solution containing a catalytic elementto the semiconductor film of prior to crystallization, crystal growthmay be by laser light irradiation instead of SPC.

This embodiment can be implemented in combination with Embodiments 1–4.

(Embodiment 6)

This embodiment explains the form of a laser beam combined bysuperposing together a plurality of laser beams.

FIG. 16A shows an example of a laser beam form on a subject to beprocessed in the case that laser light is oscillated from a plurality oflaser oscillators without a slit and n laser light energy densitydistribution in a major-axis. The laser beam shown FIG. 16A is ellipticin form. Incidentally, in the invention, the laser beam form of laserlight oscillated from the laser oscillator is not limited to theelliptic.

The laser beam form is different depending on a laser kind and can beformed by an optical system. For example, the laser light emitted froman XeCl excimer laser (wavelength 308 nm, pulse width 30 ns) L3308 byLambda is rectangular in form having 10 mm×30 mm (each, width at halfmaximum in beam profile). The laser light emitted from a YAG laser iscircular in form if a rod is cylindrical and rectangular in form if itis a slab type. By further forming such laser light by an opticalsystem, a desired size of laser light can be formed.

The laser light has an energy density distribution increasing toward anelliptic center O. In this manner, the laser beam has an energy densityin a center axis direction following the Gaussian distribution, whereinthe region is narrow where energy density is to be determined uniform.

FIG. 16B shows a laser beam form when the laser light having a laserbeam of FIG. 16A is combined together. Although FIG. 16B shows the casethat four laser-light laser beams are superposed together to form onelinear laser beam, the number of laser beams superposed is not limitedto that.

As shown in FIG. 16B, the laser beams of laser light are combinedtogether by partly superposed one over another with their major axesplaced in coincidence, thereby being formed into one laser beam 360.Note that, hereinafter, a straight line obtained by connecting theellipse centers O is assumed to be a center axis of the laser beam 360.

FIG. 16B shows the laser-light energy density distribution in acenter-axis y-direction of a combined laser beam. Energy density isadded on in the overlapped areas of the uncombined laser beams. Forexample, adding the energy densities E1 and E2 together of theoverlapped beams as shown in the figure, it becomes nearly equal to apeak value E3 of beam energy density. Thus, energy density is flattenedbetween the elliptic centers O.

Incidentally, the addition of E1 and E2 together, ideally, equals to E3,practically an equal value is not necessarily obtainable. It is possiblefor the designer to appropriately set an allowable range of deviationbetween the added value of E1 and E2 and the value of E 3.

With the use of a single laser beam, the energy density distributionfollows the Gaussian distribution. Accordingly, it is difficult toirradiate an even energy density of laser light to the entire of asemiconductor film contacting with the flat region of insulating film ora part to be made into an island. However, as can be seen from FIG. 16B,by superposing together a plurality of laser light to mutuallycompensate for the regions low in energy density, the region having auniform energy density is broadened rather than the single use thereofwithout superposing a plurality of laser light. This can efficientlyenhance the crystallinity of a semiconductor film.

FIG. 17 shows an energy density distribution, determined by computation,on B–B′ and C–C′ in FIG. 16B. Note that FIG. 17 is with reference to theregion satisfying an energy density of 1/e² of a peak value of anuncombined laser beam. When the uncombined laser beam assumably has alength in minor axis direction of 37 μm and a length in major axisdirection of 410 μm and a center-to-center distance of 192 μm, theenergy densities on B–B′ and C—C′ have respective distributions as shownin FIG. 17A and FIG. 17B. Although the one on B–B′ is somewhat smallerthan the one on C–C′, these can be considered to be substantially thesame in magnitude. The combined laser beam, in a region satisfying anenergy density of 1/e² of a peak value of an uncombined laser beam, canbe considered as linear in form.

FIG. 18A shows an energy distribution of a combined laser beam. Theregion shown at 361 is a region where energy density is to be determineduniform while the region shown at 362 is a region having a low energydensity. In FIG. 18, it is assumed that the laser beam has a length in acenter axis direction of W_(TBW) while the region 361 having a uniformenergy density has a length in a center axis direction of W_(max). AsW_(TBW) increases greater as compared to W_(max), the ratio of theregion 362 uneven in energy density not to be used in crystallizing asemiconductor film increases relatively to the region 361 uniform inenergy density to be used in crystallization. The semiconductor filmirradiated only by the region 362 uneven in energy density has finecrystals, thus being not well in crystallinity. Consequently, therearises a necessity to define a layout of scanning route and insulatingfilm depression-projection such that the region of semiconductor film tobe made into an island is not superposed with only the region 362. Thisrestriction increases furthermore as the ratio of region 362 to region361 increases. Accordingly, it is effective to use a slit to preventonly the region 362 uneven in energy density from being irradiated tothe semiconductor film formed on the insulating film depression part orprojection part, in respect of decreasing the restriction occurring uponproviding a layout of scanning route and insulating filmdepression-projection.

This embodiment can be implemented by combining with Embodiments 1through 5.

(Embodiment 7)

This embodiment describes an optical system of laser irradiationapparatus used in the present invention, and the positional relationbetween a slit and the optical system.

FIG. 19 shows an optical system for synthesizing four laser beams toobtain one laser beam. The optical system shown in FIG. 19 has sixcylindrical lenses 417 to 422. Four laser beams entering the opticalsystem from the directions indicated by the arrows separately enter thefour cylindrical lenses 419 to 422. Two laser beams shaped by thecylindrical lenses 419 and 421 reach the cylindrical lens 417, whichmodifies the shapes of the laser beams. The laser beams travel through aslit 424 to irradiate a processing object 423. On the other hand, twolaser beams shaped by the cylindrical lenses 420 and 422 reach thecylindrical lens 418, which modifies the shapes of the laser beams. Thelaser beams travel through the slit 424 to irradiate the processingobject 423.

The laser beams on the processing object 423 partially overlap oneanother for synthesization, thereby forming one laser beam.

The focal length and incident angle of each lens can be set atdesigner's discretion. However, the focal length of the cylindricallenses 417 and 418 which are the closest to the processing object 423 isset shorter than the focal length of the cylindrical lenses 419 to 422.For example, the focal length of the cylindrical lenses 417 and 418which are the closest to the processing object 423 is set to 20 mmwhereas the focal length of the cylindrical lenses 419 to 422 is set to150 mm. In this embodiment, the lenses are arranged such that laserbeams enter the processing object 423 from the cylindrical lenses 417and 418 at an incident angle of 25° and laser beams enter thecylindrical lenses 417 and 418 from the cylindrical lenses 419 to 422 atan incident angle of 10°. In order to avoid return light and irradiateuniformly, the incident angle at which laser light enters the substrateis kept at an angle larger than 0°, desirably, 5 to 30°.

In the example shown in FIG. 19, four laser beams are synthesized. Inthis case, four cylindrical lenses respectively associated with fourlaser oscillators and two cylindrical lenses associated with the fourcylindrical lenses are provided. The number of laser beams synthesizedis not limited to 4. It is sufficient if the number of laser beamssynthesized is equal to or more than 2 and equal to or less than 8. Whenn (n=2, 4, 6, 8) laser beams are synthesized, n cylindrical lensesrespectively associated with n laser oscillators and n/2 cylindricallenses associated with the n cylindrical lenses are provided. When n(n=3, 5, 7) laser beams are synthesized, n cylindrical lensesrespectively associated with n laser oscillators and (n+1)/2 cylindricallenses associated with the n cylindrical lenses are provided.

When five or more laser beams are synthesized, the fifth and thefollowing laser beams desirably irradiate a substrate from the oppositeside of the substrate, taking into consideration where to place theoptical system, interference, and the like. In this case, another slitis needed on the opposite side of the substrate. Also, the substrate hasto be transmissive.

In order to prevent light from traveling back its light path (returnlight), the incident angle at which laser light enters the substrate isdesirably kept at an angle larger than 0° and smaller than 90°.

A plane which is perpendicular to the irradiated face and which includesa shorter side of the rectangular shape of each beam beforesynthesization, or a longer side thereof, is defined as an incidentplane. When the length of the shorter side or longer side included inthe incident plane is given as W, and the thickness of a substrate whichis transmissive of the laser light and which is set on the irradiatedface is given as d, an incident angle θ of the laser light desirablysatisfies θ≧arctan (W/2d) to achieve uniform laser light irradiation.This has to be true in each laser light before synthesization. If thetrack of this laser light is not on the incident plane, the incidentangle of the track projected onto the incident plane is deemed as θ.When laser light enters the substrate at this incident angle θ,interference between light reflected at the front side of the substrateand reflected light from the back side of the substrate can be avoidedto give the substrate uniform laser beam irradiation. The premise of theabove discussion is that the refractive index of the substrate is 1. Inpractice, the refractive index of the substrate is often around 1.5, andthe angle calculated taken this fact into account is larger than theangle calculated in the above discussion. However, the energy of a beamspot is attenuated at its ends in the longitudinal direction andinfluence of interference is small in these portions. Therefore enoughinterference attenuation effect can be obtained with the valuecalculated in the above discussion. The above-mentioned inequality of θdoes not apply to those substrates that are transmissive to a laserbeam.

An optical system of laser irradiation apparatus used in the presentinvention can have other structures than the one shown in thisembodiment.

This embodiment can be combined with Embodiments 1 through 6.

(Embodiment 8)

The laser light having a laser beam in an elliptic form has an energydensity distribution perpendicular to a scanning direction following theGaussian distribution. Consequently, the ratio of a low energy densityregion to the entire is higher as compared to the laser light having arectangular or linear laser beam. Accordingly, in the invention, thelaser beam of laser light is desirably rectangular or linearcomparatively uniform in energy density distribution.

The representative gas laser, for obtaining a rectangular or linearlaser beam, is an excimer laser while the representative solid laser isa slab laser. This embodiment explains a slab laser.

FIG. 20A shows an example of a laser oscillator structure of a slabtype. The slab-type laser oscillator of FIG. 20A has a rod 7500, areflection mirror 7501, an output mirror 7502 and a cylindrical lens7503.

In case an excitation light is irradiated to the rod 7500, laser lighttravels through a zigzag optical path and emits toward the reflectionmirror 7501 or emission mirror 7502. The laser light emitted toward thereflection mirror 7501 is reflected thereon and again enters the rod7500, then emitting toward the emission mirror 7502. The rod 7500 is ofa slab type using a plate-like slab medium to form a comparatively longrectangular or linear laser beam upon emission. The emitted laser light,in the cylindrical lens 7503, is formed smaller in its laser beam formand emitted at the laser oscillator.

FIG. 20B shows a slab-type laser oscillator structure different fromthat showed in FIG. 20A. In FIG. 20B, a cylindrical lens 7504 is addedto the laser oscillator of FIG. 20A to control a laser beam length bythe cylindrical lens 7504.

Incidentally, with a coherent length of 10 cm or longer, preferably 1 mor longer, the laser beam can be reduced in form furthermore.

In order to prevent the rod 7500 from excessively rising in temperature,temperature control means may be provided, e.g. circulating a coolingwater.

FIG. 20C shows an embodiment of a cylindrical lens form. 7509 is acylindrical lens of this embodiment fixed by a holder 7510. Thecylindrical lens 7509 has a form that a cylindrical surface and arectangular flat surface are opposed to each other, wherein the twogenerating lines of the cylindrical surface and the two sides of theopposed rectangle are all in parallel with one another. The twosurfaces, formed by the two lines of cylindrical surface and theparallel two lines, intersect with the rectangular flat surface at anangle greater than 0 degree and smaller than 90 degrees. In this manner,the two surfaces formed with the two parallel sides intersect with therectangular flat surface at an angle of smaller than 90 degrees, wherebythe focal length can be shortened as compared to that at 90 degrees orgreater. This can further reduce the form of laser beam and approximateit to a linear form.

This embodiment can be implemented by combining with Embodiments 1through 7.

(Embodiment 9)

This embodiment explains a relationship between a center-to-centerdistance of laser beams and an energy density when laser beams aresuperposed one over another.

FIG. 21 shows an energy density distribution of each laser beam in acenter axis direction by the solid line and an energy densitydistribution of a combined laser beam by the dotted line. The energydensity value of a laser beam in a center axis direction of a laser beamgenerally follows the Gaussian distribution.

It is assumed that, the uncombined laser beam has a peak-to-peakdistance X when a distance in a center axis direction is taken as 1 thatis satisfying an energy density equal to or greater than 1/e² of a peakvalue. Meanwhile, in a combined laser beam, the increase amount of peakvalue is assumably taken as Y with respect to an average value of a peakvalue and valley value of after combination. FIG. 22 shows arelationship between X and Y determined on simulation. Note that Y inFIG. 22 is expressed by percentage.

In FIG. 22, an energy difference Y is expressed by an approximateexpression as given in the following Equation 1.Y=60−293X+340X ² (X: assumed to be greater one of twosolutions)  [Equation 1]

According to Equation 1, it can be seen that X≈0.584 may be providedwhen obtaining an energy difference of approximately 5% for example.Incidentally, although ideally Y=0, there is practically a difficulty inrealizing it. There is a need for the designer to appropriately set anallowable range of energy difference Y. Although ideally Y=0, it makesthe beam spot length short. Consequently, X is preferably determinedconsidering a balance with throughput.

Explanation is now made on an allowable range of Y. FIG. 23 shows anoutput (W) distribution of YVO₄ laser with respect to a beam width in acenter axis direction in the case the laser beam has an elliptic form.The hatched region is an output energy range required to obtainfavorable crystallinity. It can be seen that the output energy ofcombined laser light is satisfactorily within a range of 3.5–6 W.

When the output energy maximum and minimum values of a beam spot aftercombination are fallen, to a full limit, within the output energy rangerequired to obtain favorable crystallinity, the energy difference Y forfavorable crystallinity assumes to be the maximum. Accordingly, in thecase of FIG. 23, the energy difference Y is +26.3%. It can be seen thatfavorable crystallinity is to be obtained provided that the energydifference Y falls within the foregoing range.

Incidentally, the output energy range for favorable crystallinity variesdepending upon to what extent crystallinity is to be determinedfavorable. Further, because output energy distribution changes dependingon a laser beam form, the allowable range of energy difference Y is notnecessarily limited to the foregoing value. The designer is required toappropriately define an output energy range required to obtain favorablecrystallinity and set an allowable range of energy difference Y from anoutput energy distribution of a laser to be used.

This embodiment can be implemented in combination with Embodiments 1–8.

(Embodiment 10)

The present invention can be applied to various semiconductor devices. Amode of a display panel manufactured in accordance with Embodiments 1through 9 will be described with reference to FIGS. 24 and 25.

In FIG. 24, a substrate 901 is provided with a pixel portion 902, gatesignal side driving circuits 901 a and 901 b, a data signal side drivingcircuit 901 c, an input/output terminal portion 908, and a wire or groupof wires 904. A shield pattern 905 may partially overlap the gate signalside driving circuits 901 a and 901 b and the data signal side drivingcircuit 901 c, as well as the wire or group of wires 904 for connectingthe driving circuits with the input/output terminal portion 908. In thisway, the area of the frame region (the region surrounding the pixelportion) of the display panel can be reduced. An FPC 903 is fixed to theinput/output terminal portion 908.

The present invention can be used in active elements constituting thepixel portion 902, the gate signal side driving circuits 901 a and 901b, and the data signal side driving circuit 901 c.

FIG. 25 shows an example of the structure of one pixel in the pixelportion 902 shown in FIG. 24. The pixel has TFTs 801 to 803, which are aswitching TFT, a reset TFT, and a driving TFT, respectively, forcontrolling a light emitting element or liquid crystal element of thepixel.

These TFTs have active layers 812 to 814. Each of the active layers isplaced between one edge of a depression portion 810 or 811 of theinsulating film formed below the active layers and the midpoint betweenthe one edge and the other edge of the depression portion. Gate wires815 to 817 are formed in a layer above the active layers 812 to 814. Apassivation film and a planarization film are formed on the gate wires.A data line 819, a power supply line 820, other various wires 821 and822, and a pixel electrode 823 are formed on the passivation film andthe planarization film.

This embodiment uses for the TFTs islands formed on the bottom ofdepression portions. Instead, islands formed on the top of projectionportions may be used for the TFTs.

This embodiment can be combined freely with Embodiments 1 through 9.

(Embodiment 11)

The semiconductor device equipped with the TFT formed by the presentinvention can be applied to various electronic apparatuses. Examples ofthe electronic apparatuses are portable information terminals(electronic books, mobile computers, cellular phones, or the like),video cameras, digital cameras, personal computers, TV receivers,cellular phones, projection display apparatuses, or the like. Specificexamples of these electronic apparatuses are shown in FIGS. 26A to 26G.

FIG. 26A shows a display apparatus, which is composed of a case 2001, asupport base 2002, a display unit 2003, speaker units 2004, a videoinput terminal 2005, etc. The display apparatus of the present inventionis completed by using the semiconductor device of the present inventionto the display unit 2003. Since the light emitting device having thelight emitting element is self-luminous, the device does not need backlight and can make a thinner display unit than liquid crystal displaydevices. The display device refers to all display devices for displayinginformation, including ones for personal computers, for TV broadcastingreception, and for advertisement.

FIG. 26B shows a digital still camera, which is composed of a main body2101, a display unit 2102, an image receiving unit 2103, operation keys2104, an external connection port 2105, a shutter 2106, etc. The digitalstill camera of the present invention is completed by using thesemiconductor device of the present invention to the display unit 2102.

FIG. 26C shows a notebook personal computer, which is composed of a mainbody 2201, a case 2202, a display unit 2203, a keyboard 2204, anexternal connection port 2205, a pointing mouse 2206, etc. The notebookpersonal computer of the present invention is completed by using thesemiconductor device of the present invention to the display unit 2203.

FIG. 26D shows a mobile computer, which is composed of a main body 2301,a display unit 2302, a switch 2303, operation keys 2304, an infraredport 2305, etc. The light emitting device manufactured in accordancewith the present invention can be applied to the display unit 2302. Themobile computer of the present invention is completed by using thesemiconductor device of the present invention to the display unit 2302.

FIG. 26E shows a portable image reproducing device equipped with arecording medium (a DVD player, to be specific). The device is composedof a main body 2401, a case 2402, a display unit A 2403, a display unitB 2404, a recording medium (DVD or the like) reading unit 2405,operation keys 2406, speaker units 2407, etc. The display unit A 2403mainly displays image information whereas the display unit B 2404 mainlydisplays text information. The light emitting device manufactured inaccordance with the present invention can be applied to the displayunits A 2403 and B 2404. The portable image reproducing device of thepresent invention is completed by using the semiconductor device of thepresent invention to the display units A 2403 and B 2404.

FIG. 26F shows a goggle type display (head mounted display), which iscomposed of a main body 2501, display units 2502, and arm units 2503.The goggle type display of the present invention is completed by usingthe semiconductor device of the present invention to the display units2502.

FIG. 26G shows a video camera, which is composed of a main body 2601, adisplay unit 2602, a case 2603, an external connection port 2604, aremote control receiving unit 2605, an image receiving unit 2606, abattery 2607, an audio input unit 2608, operation keys 2609, eye pieceportion 2610 etc. The video camera of the present invention is completedby using the semiconductor device of the present invention to thedisplay unit 2602.

FIG. 26H shows a cellular phone, which is composed of a main body 2701,a case 2702, a display unit 2703, an audio input unit 2704, an audiooutput unit 2705, operation keys 2706, an external connection port 2707,an antenna 2708, etc. The light emitting device manufactured inaccordance with the present invention can be applied to the display unit2703. If the display unit 2703 displays white letters on blackbackground, the cellular phone consumes less power. The cellular phoneof the present invention is completed by using the semiconductor deviceof the present invention to the display unit 2703.

As described above, the application range of the present invention is sowide that it is applicable to electric apparatuses of any field. Thisembodiment can be operated by combining with any structure shown inEmbodiments 1 through 10.

(Embodiment 12)

This embodiment describes the structure of a semiconductor device of thepresent invention. FIG. 27 is a sectional view of a semiconductor deviceof this embodiment.

A first insulating film 701 having projection portions 701 a and 701 bis formed on a substrate 700. A first TFT 702 is formed on the firstinsulating film 701. An island of the first TFT 702 is formed on thefirst insulating film 701 between one edge of a depression portion andthe midpoint between the one edge and the other edge of the depressionportion. This depression portion is created by the projection portions701 a and 701 b.

A first interlayer insulating film 703 is formed to cover the first TFT702. On the first interlayer insulating film 703, a first connectionwire 705 and a wire 704 are formed. The wire 704 is electricallyconnected to the first TFT 702.

A second interlayer insulating film 706 is formed to cover the wire 704and the first connection wire 705. The second interlayer insulating film706 is an inorganic insulating film. If the top face of the secondinterlayer insulating film is polished by chemical mechanical polishing(CMP), a second insulating film formed later can have more level surfaceand the crystallinity of a semiconductor film to be formed on the secondinsulating film and crystallized by laser light can be enhanced.

A second insulating film 707 is formed on the second interlayerinsulating film 706. The second insulating film 707 has a projectionportion 707 a. A second TFT 708 is formed on the second insulating film707. An island of the second TFT 708 is formed on the second insulatingfilm 707 between one edge of a depression portion and the midpointbetween the one edge and the other edge (not shown in the drawing) ofthe depression portion. This depression portion is created by theprojection portion 707 a.

A third interlayer insulating film 709 is formed to cover the second TFT708. On the third interlayer insulating film 709, a second connectionwire 711 and a wire 710 are formed. The wire 710 is electricallyconnected to the second TFT 708. An embedded wire (plug) 712 is formedbetween the first connection wire 705 and the second connection wire 711by Damascene process or the like.

A fourth interlayer insulating film 713 is formed to cover the wire 710and the second connection wire 711.

In this embodiment, the first TFT 702 and the second TFT 708 overlapeach other with an interlayer insulating film sandwiched therebetween toform a so-called stack structure. The stack structure TFTs of thisembodiment can be used to build a CPU using an LSI, memory devices(e.g., SRAM) of various logic circuits, a counter circuit, a frequencydivider circuit, etc.

This embodiment uses for the TFTs islands formed on the bottom ofdepression portions. Instead, islands formed on the top of projectionportions may be used for the TFTs.

This embodiment can be combined freely with Embodiments 1 through 11.

(Embodiment 13)

This embodiment describes the energy density distribution of a linearlaser beam that is obtained by synthesizing plural elliptical laserbeams.

FIG. 28 shows the energy density distribution in 1/e² width of a laserbeam obtained by overlapping two elliptical laser beams each measuring400 μm in major axis and 40 μm in minor axis. Measurements in the graphare all in mm (unit). The distance between centers of adjacent beams is0.255 mm.

In addition, FIG. 29 shows the energy density distribution in 1/e² widthof a laser beam obtained by overlapping two elliptical laser beams eachmeasuring 400 μm in major axis and 40 μm in minor axis. Measurements inthe graph are all in mm (unit). The distance between centers of adjacentbeams is 0.255 mm.

In addition, FIG. 30 shows the energy density distribution in 1/e² widthof a laser beam obtained by overlapping four elliptical laser beams eachmeasuring 400 μm in major axis and 40 μm in minor axis. Measurements inthe graph are all in mm (unit). The distance between centers of adjacentbeams is 0.215 mm.

In an elliptical laser beam, the energy density distribution in thecenter line direction matches Gaussian distribution. On the other hand,a laser beam obtained by overlapping plural elliptical laser beams hasan energy density distribution in the center line direction which formsa waveform above a certain level as shown in FIGS. 28, 29, and 30.Unlike an elliptical laser beam, it can be said that the energy densitydistribution in the center line direction of a laser beam obtained bysynthesizing elliptical laser beams is relatively uniform and linear.

The use of such laser beam having a linear energy density distributionin the present invention makes it possible to form an island that hasuniform crystallinity.

This embodiment can be combined freely with Embodiments 1 through 12.

(Embodiment 14)

This embodiment gives a description on the concentration of oxygen,nitrogen, carbon, and boron taken into a semiconductor film irradiatedwith continuous wave laser light.

First, an amorphous silicon film is formed to a thickness of 1500 Å onan insulating film that is formed of silicon oxynitride. A nickelacetate solution is applied to the amorphous silicon film and the filmis heated at 500 to 650° C. Continuous wave laser light is then used tocrystallize the film and obtain a crystalline silicon film (poly-Si).The laser light irradiation is conducted in the air in a clean room. Athin oxide film is naturally formed (natural oxide film) on the surfaceof the crystalline silicon film. Then an amorphous silicon film isformed to cover the crystalline silicon film and the natural oxide film.

In this state, secondary ion mass spectroscopy (SIMS) is performed onthe film. The atomic percentage profiles of oxygen, nitrogen, carbon,and boron are shown in FIGS. 31 to 34, respectively.

The oxygen concentration profile in the crystalline or amorphous siliconfilm is measured by SIMS and the results are shown in FIG. 31. The axisof ordinate shows the atomic percentage of oxygen and the axis ofabscissa shows the depth from the sample surface. The solid lineindicates the oxygen concentration of when laser light irradiationprocess is carried out, and the dashed line indicates the oxygenconcentration of when laser light irradiation process is not carriedout. The graph also shows the ionic strength of silicon with the axis ofabscissa indicating the depth from the sample surface. The oxygenconcentration after laser light irradiation is 2×10¹⁹ atoms/cm³ orlower. As FIG. 31 shows, the oxygen concentration in the silicon film isincreased by laser light irradiation.

The nitrogen concentration profile in the crystalline or amorphoussilicon film is measured by SIMS and the results are shown in FIG. 32.The axis of ordinate shows the atomic percentage of nitrogen and theaxis of abscissa shows the depth from the sample surface. The solid lineindicates the nitrogen concentration of when laser light irradiationprocess is carried out, and the dashed line indicates the nitrogenconcentration of when laser light irradiation process is not carriedout. The graph also shows the ionic strength of silicon with the axis ofabscissa indicating the depth from the sample surface. The nitrogenconcentration after laser light irradiation is 1×10¹⁹ atoms/cm³ orlower. As FIG. 32 shows, the nitrogen concentration in the silicon filmis increased by laser light irradiation.

The carbon concentration profile in the crystalline or amorphous siliconfilm is measured by SIMS and the results are shown in FIG. 33. The axisof ordinate shows the atomic percentage of carbon and the axis ofabscissa shows the depth from the sample surface. The solid lineindicates the carbon concentration of when laser light irradiationprocess is carried out, and the dashed line indicates the carbonconcentration of when laser light irradiation process is not carriedout. The graph also shows the ionic strength of silicon with the axis ofabscissa indicating the depth from the sample surface. The carbonconcentration after laser light irradiation is 5×10¹⁸ atoms/cm³ orlower. As FIG. 33 shows, the carbon concentration in the silicon film isincreased by laser light irradiation.

The boron concentration profile in the crystalline or amorphous siliconfilm is measured by SIMS and the results are shown in FIG. 34. The axisof ordinate shows the atomic percentage of boron and the axis ofabscissa shows the depth from the sample surface. The solid lineindicates the boron concentration of when laser light irradiationprocess is carried out, and the dashed line indicates the boronconcentration of when laser light irradiation process is not carriedout. The graph also shows the ionic strength of silicon with the axis ofabscissa indicating the depth from the sample surface. In FIG. 34, itseems that the boron concentration in the silicon film is slightlyincreased by laser light irradiation. The boron concentration is belowthe lowest level detectable by SIMS before and after laser irradiationanyway and the boron content in the film is very minute.

(Embodiment 15)

This embodiment describes the shape of an insulating film and a relationbetween it and the thickness of a semiconductor film formed on theinsulating film.

FIG. 35A shows an insulating film 950 having projection portions 950 aand a semiconductor film 951 that is formed on the insulating film. Thesemiconductor film 951 shown here has already been crystallized by laserlight.

As shown in FIG. 35A, a thickness Ht of the semiconductor film 951 onthe projection portions 950 a is smaller than a thickness Hb of thesemiconductor film 951 on a depression portion between the projectionportions 950 a. This is supposedly because the semiconductor filmtemporarily melted by laser light irradiation moves into the depressionportion. Therefore it is considered that the surface of thesemiconductor film 951 is leveled to a certain degree through laserlight irradiation.

FIG. 35B shows an insulating film 960 having projection portions 960 aand a semiconductor film 961 which is formed on the insulating film andwhich has a flat surface. In contrast to FIG. 35A where thesemiconductor film 951 has depression and projection on its surface, thesurface of the semiconductor film in FIG. 35B is leveled by laser lightirradiation.

FIG. 35C is a sectional view of an insulating film in which a width Wtof a projection portion is larger than a width Wb of a depressionportion in the direction perpendicular to the laser light scanningdirection. When a portion of a semiconductor film that is on aprojection portion and has excellent crystallinity is used as an activelayer of a TFT, the width Wt of a projection portion is preferablylarger than the width Wb of a depression portion as shown in FIG. 35Cbecause this reduces restrictions in layout of an island.

This embodiment can be combined freely with Embodiments 1 through 14.

In the present invention, a semiconductor film crystallized by laserlight is patterned to remove a portion of the film around the midpointbetween edges of a depression portion or projection portion. Theremaining portion between one edge of a depression portion or projectionportion and the midpoint between the one edge and the other edge of thedepression portion or projection portion, which has fewer grainboundaries and therefore has excellent crystallinity, is effectivelyused as an active layer of a TFT. This makes it possible to avoidforming a grain boundary in a channel formation region of a TFT, therebypreventing grain boundaries from lowering the mobility of the TFTgreatly, from lowering ON current, and from increasing OFF current. Howfar from an edge of a depression portion or projection portion is to beremoved by patterning can be decided at designer's discretion.

The present invention runs laser light so as to obtain at least theminimum degree of crystallization of a portion that has to becrystallized, instead of irradiating the entire semiconductor film withlaser light. This structure saves time for laser irradiation of portionsthat are removed by patterning after crystallization of thesemiconductor film and thereby greatly shortens the processing time persubstrate.

The crystallinity of a semiconductor film can be enhanced moreefficiently when plural laser beams are overlapped to supplement oneanother's low energy density portions than when using a single laserbeam.

The position of a grain boundary in a semiconductor film formed on aninsulating film may be controlled by forming depression and projectionportions on a substrate itself through etching, instead of formingdepression and projection portions on the insulating film.

1. A semiconductor device comprising: an insulating film having adepression portion and a projection portion; and a thin film transistorhaving a channel formation region that is placed only between a centerand an edge of the depression portion of the insulating film.
 2. Thesemiconductor device according to claim 1, wherein a bottom of thedepression portion of the insulating film is wider than a top of theprojection portion in a direction perpendicular to the longitudinaldirection.
 3. The semiconductor device according to claim 1, wherein thedepression portion of the insulating film is formed not forming aninsulating film comprising a silicon nitride film or a siliconoxynitride film.
 4. The semiconductor device according to claim 1,wherein an oxygen concentration in the channel formation region is2×10¹⁹ atoms/cm³ or less.
 5. The semiconductor device according to claim1, wherein a carbon concentration in the channel formation region is1×10¹⁹ atoms/cm³ or less.
 6. The semiconductor device according to claim1, wherein a nitrogen concentration in the channel formation region is1×10¹⁹ atoms/cm³ or less.
 7. The semiconductor device according to claim1, wherein the depression portion is a rectangular or stripe pattern. 8.A semiconductor device comprising: an insulating film having adepression portion and a projection portion; and a thin film transistorhaving a channel formation region over the depression portion that isnot placed over a midpoint of the depression portion of the insulatingfilm.
 9. The semiconductor device according to claim 8, wherein a bottomof the depression portion of the insulating film is wider than a top ofthe projection portion in a direction perpendicular to the longitudinaldirection.
 10. The semiconductor device according to claim 8, whereinthe depression portion of the insulating film is formed not forming aninsulating film comprising a silicon nitride film or a siliconoxynitride film.
 11. The semiconductor device according to claim 8,wherein an oxygen concentration in the channel formation region is2×10¹⁹ atoms/cm³ or less.
 12. The semiconductor device according toclaim 8, wherein a carbon concentration in the channel formation regionis 1×10¹⁹ atoms/cm³ or less.
 13. The semiconductor device according toclaim 8, wherein a nitrogen concentration in the channel formationregion is 1×10¹⁹ atoms/cm³ or less.
 14. The semiconductor deviceaccording to claim 8, wherein the depression portion is a rectangular orstripe pattern.
 15. A semiconductor device comprising: an insulatingfilm having a depression portion and a projection portion; and a thinfilm transistor having a channel formation region that is placed onlybetween a center and an edge of the projection portion of the insulatingfilm.
 16. The semiconductor device according to claim 15, wherein a topof the projection portion is wider than a bottom of the depressionportion in a direction perpendicular to the longitudinal direction. 17.The semiconductor device according to claim 15, wherein the projectionportion of the insulating film is formed by a first insulating film anda second insulating film, the first insulating film comprising a siliconoxide film or a silicon oxynitride film, and the second insulating filmcomprising a silicon nitride film or a silicon oxynitride film.
 18. Thesemiconductor device according to claim 15, wherein the projectionportion of the insulating film is trapezoidal in section in a directionperpendicular to the longitudinal direction.
 19. The semiconductordevice according to claim 15, wherein an oxygen concentration in thechannel formation region is 2×10¹⁹ atoms/cm³ or less.
 20. Thesemiconductor device according to claim 15, wherein a carbonconcentration in the channel formation region is 1×10¹⁹ atoms/cm³ orless.
 21. The semiconductor device according to claim 15, wherein anitrogen concentration in the channel formation region is 1×10¹⁹atoms/cm³ or less.
 22. The semiconductor device according to claim 15,wherein the projection portion is a rectangular or stripe pattern.
 23. Asemiconductor device comprising: an insulating film having a depressionportion and a projection portion; and a thin film transistor having achannel formation region over the projection portion that is not placedover a midpoint of the projection portion of the insulating film. 24.The semiconductor device according to claim 23, wherein a top of theprojection portion is wider than a bottom of the depression portion in adirection perpendicular to the longitudinal direction.
 25. Thesemiconductor device according to claim 23, wherein the projectionportion of the insulating film is formed by a first insulating film anda second insulating film, the first insulating film comprising a siliconoxide film or a silicon oxynitride film, and the second insulating filmcomprising a silicon nitride film or a silicon oxynitride film.
 26. Thesemiconductor device according to claim 23, wherein the projectionportion of the insulating film is trapezoidal in section in a directionperpendicular to the longitudinal direction.
 27. The semiconductordevice according to claim 23, wherein an oxygen concentration in thechannel formation region is 2×10¹⁹ atoms/cm³ or less.
 28. Thesemiconductor device according to claim 23, wherein a carbonconcentration in the channel formation region is 1×10¹⁹ atoms/cm³ orless.
 29. The semiconductor device according to claim 23, wherein anitrogen concentration in the channel formation region is 1×10¹⁹atoms/cm³ or less.
 30. The semiconductor device according to claim 23,wherein the projection portion is a rectangular or stripe pattern.
 31. Asemiconductor device comprising: an insulating film having a rectangularor stripe pattern depression portion and projection portion; and a thinfilm transistor having a channel formation region that is placed onlybetween a center and an edge of the depression portion of the insulatingfilm.
 32. The semiconductor device according to claim 31, wherein abottom of the depression portion of the insulating film is wider than atop of the projection portion in a direction perpendicular to thelongitudinal direction.
 33. The semiconductor device according to claim31, wherein the depression portion and the projection portion of theinsulating film are formed by a first insulating film and a secondinsulating film, the first insulating film comprising a silicon oxidefilm or a silicon oxynitride film, and the second insulating filmcomprising a silicon nitride film or a silicon oxynitride film forming arectangular or stripe pattern on the first insulating film.
 34. Thesemiconductor device according to claim 31, wherein the projectionportion of the insulating film is trapezoidal in section in a directionperpendicular to the longitudinal direction.
 35. The semiconductordevice according to claim 31, wherein an oxygen concentration in thechannel formation region is 2×10¹⁹ atoms/cm³ or less.
 36. Thesemiconductor device according to claim 31, wherein a carbonconcentration in the channel formation region is 1×10¹⁹ atoms/cm³ orless.
 37. The semiconductor device according to claim 31, wherein anitrogen concentration in the channel formation region is 1×10¹⁹atoms/cm³ or less.
 38. A semiconductor device comprising: an insulatingfilm having a rectangular or stripe pattern depression portion andprojection portion; and a thin film transistor having a channelformation region over the depression portion that is not placed over amidpoint of the depression portion of the insulating film.
 39. Thesemiconductor device according to claim 38, wherein a bottom of thedepression portion of the insulating film is wider than a top of theprojection portion in a direction perpendicular to the longitudinaldirection.
 40. The semiconductor device according to claim 38, whereinthe depression portion and the projection portion of the insulating filmare formed by a first insulating film and a second insulating film, thefirst insulating film comprising a silicon oxide film or a siliconoxynitride film, and the second insulating film comprising a siliconnitride film or a silicon oxynitride film forming a rectangular orstripe pattern on the first insulating film.
 41. The semiconductordevice according to claim 38, wherein the projection portion of theinsulating film are trapezoidal in section in a direction perpendicularto the longitudinal direction.
 42. The semiconductor device according toclaim 38, wherein an oxygen concentration in the channel formationregion is 2×10¹⁹ atoms/cm³ or less.
 43. The semiconductor deviceaccording to claim 38, wherein a carbon concentration in the channelformation region is 1×10¹⁹ atoms/cm³ or less.
 44. The semiconductordevice according to claim 38, wherein a nitrogen concentration in thechannel formation region is 1×10¹⁹ atoms/cm³ or less.
 45. Asemiconductor device comprising: an insulating film having a rectangularor stripe pattern depression portion and projection portion; and a thinfilm transistor having a channel formation region that is placed onlybetween a center and an edge of the projection portion of the insulatingfilm.
 46. The semiconductor device according to claim 45, wherein a topof the projection portion of the insulating film is wider than a bottomof the depression portion in a direction perpendicular to thelongitudinal direction.
 47. The semiconductor device according to claim45, wherein the depression portion and the projection portion of theinsulating film are formed by a first insulating film and a secondinsulating film, the first insulating film comprising a silicon oxidefilm or a silicon oxynitride film, and the second insulating filmcomprising a silicon nitride film or a silicon oxynitride film forming arectangular or stripe pattern on the first insulating film.
 48. Thesemiconductor device according to claim 45, wherein the projectionportion of the insulating film is trapezoidal in section in a directionperpendicular to the longitudinal direction.
 49. The semiconductordevice according to claim 45, wherein an oxygen concentration in thechannel formation region is 2×10¹⁹ atoms/cm³ or less.
 50. Thesemiconductor device according to claim 45, wherein a carbonconcentration in the channel formation region is 1×10¹⁹ atoms/cm³ orless.
 51. The semiconductor device according to claim 45, wherein anitrogen concentration in the channel formation region is 1×10¹⁹atoms/cm³ or less.
 52. A semiconductor device comprising: an insulatingfilm having a rectangular or stripe pattern depression portion andprojection portion; and a thin film transistor having a channelformation region over the projection portion that is not placed over amidpoint of the projection portion of the insulating film.
 53. Thesemiconductor device according to claim 52, wherein a top of theprojection portion of the insulating film is wider than a bottom of thedepression portion in a direction perpendicular to the longitudinaldirection.
 54. The semiconductor device according to claim 52, whereinthe depression portion and the projection portion of the insulating filmare formed by a first insulating film and a second insulating film, thefirst insulating film comprising a silicon oxide film or a siliconoxynitride film, and the second insulating film comprising a siliconnitride film or a silicon oxynitride film forming a rectangular orstripe pattern on the first insulating film.
 55. The semiconductordevice according to claim 52, wherein the projection portion of theinsulating film is trapezoidal in section in a direction perpendicularto the longitudinal direction.
 56. The semiconductor device according toclaim 52, wherein an oxygen concentration in the channel formationregion is 2×10¹⁹ atoms/cm³ or less.
 57. The semiconductor deviceaccording to claim 52, wherein a carbon concentration in the channelformation region is 1×10¹⁹ atoms/cm³ or less.
 58. The semiconductordevice according to claim 52, wherein a nitrogen concentration in thechannel formation region is 1×10¹⁹ atoms/cm³ or less.
 59. Asemiconductor device comprising: an insulating film having a depressionportion and a projection portion; a first thin film transistor having achannel formation region that is placed only between a center and anedge of the depression portion of the insulating film; and a second thinfilm transistor over the first thin film transistor.
 60. Thesemiconductor device according to claim 59, wherein bottom of thedepression portion of the insulating film is wider than a top of theprojection portion in a direction perpendicular to the longitudinaldirection.
 61. The semiconductor device according to claim 59, whereinthe depression portion of the insulating film is formed not forming aninsulating film comprising a silicon nitride film or a siliconoxynitride film.
 62. The semiconductor device according to claim 59,wherein an oxygen concentration in the channel formation region is2×10¹⁹ atoms/cm³ or less.
 63. The semiconductor device according toclaim 59, wherein a carbon concentration in the channel formation regionis 1×10¹⁹ atoms/cm³ or less.
 64. The semiconductor device according toclaim 59, wherein a nitrogen concentration in the channel formationregion is 1×10¹⁹ atoms/cm³ or less.
 65. The semiconductor deviceaccording to claim 59, wherein the depression portion is a rectangularor stripe pattern.
 66. A semiconductor device comprising: an insulatingfilm having a depression portion and a projection portion; a first thinfilm transistor having a channel formation region over the depressionportion that is not placed over a midpoint of the depression portion ofthe insulating film; and a second thin film transistor over the firstthin film transistor.
 67. The semiconductor device according to claim66, wherein a bottom of the depression portion of the insulating film iswider than a top of the projection portion in a direction perpendicularto the longitudinal direction.
 68. The semiconductor device according toclaim 66, wherein the depression portion of the insulating film isformed not forming an insulating film comprising a silicon nitride filmor a silicon oxynitride film forming a rectangular or stripe pattern.69. The semiconductor device according to claim 66, wherein an oxygenconcentration in the channel formation region is 2×10¹⁹ atoms/cm³ orless.
 70. The semiconductor device according to claim 66, wherein acarbon concentration in the channel formation region is 1×10¹⁹ atoms/cm³or less.
 71. The semiconductor device according to claim 66, wherein anitrogen concentration in the channel formation region is 1×10¹⁹atoms/cm³ or less.
 72. The semiconductor device according to claim 66,wherein the depression portion is a rectangular or stripe pattern.